1. Field of the Invention
The present invention relates to a recording apparatus that performs recording on a recording medium and a method for controlling the recording apparatus. Furthermore, the present invention relates to a control operation for conveying a recording medium and a control operation for a recording unit.
2. Description of the Related Art
Recent recording apparatuses are capable of recording high-quality images at higher speeds. To realize high-quality and high-speed image formation, there are various methods for controlling conveyance systems of recording apparatuses.
A first method is described below. As discussed in Japanese Patent Application Laid-Open No. 2002-128313, a conveyance roller includes an optical rotary encoder that generates a signal to be used for a conveyance control operation. The conveyance control operation includes controlling the amount of conveyance and the speed of a conveyance roller.
FIG. 2 illustrates a conventional recording apparatus that includes a conveyance roller equipped with a rotary encoder. The recording apparatus illustrated in FIG. 2 includes a plurality of line-type recording heads 1 aligned in a predetermined conveyance direction along which a recording medium 2 is conveyed. A conveyance belt 3, entrained around a conveyance roller 4 and two driven rollers 5, conveys the recording medium 2. When the conveyance roller 4 rotates, the conveyance belt 3 moves in a predetermined direction while the driven rollers 5 tighten the conveyance belt 3. A conveyance roller shaft 6 is fixed to the conveyance roller 4. A conveyance motor 7 drives the conveyance roller shaft 6. A rotary encoder 8 is provided on the conveyance roller shaft 6.
A head driving control unit 10A controls each recording head 1 that operates in synchronization with conveyance timing of the recording medium 2. A motor driving unit 12A controls the conveyance motor 7, which rotates to drive the conveyance roller shaft 6. The rotary encoder 8 outputs a rotation pulse signal 201. The rotation pulse signal 201 is a periodic pulse signal representing the rotation of the conveyance roller 4. A timing generation unit 11A generates a recording timing reference signal 202 based on the rotation pulse signal 201.
When the recording apparatus performs a recording operation, the motor driving unit 12A drives the conveyance motor 7 to enable the conveyance belt 3 to travel at a constant speed. The head driving control unit 10A drives respective recording heads 1 to perform recording on the recording medium 2.
FIG. 3A is a plan view of the conveyance belt 3. To simplify description, FIG. 3A illustrates only one recording head 1, which forms an image of character “A” on the recording medium 2. The number of recorded dots forming a character image on the recording medium 2 is 12 in the conveyance direction and 9 in a direction perpendicular to the conveyance direction (generally, referred to as “conveyance width direction”). The leftmost dot array (a vertical dot array positioned at the left end in the conveyance direction) is referred to as “1st LINE”, and the next dot array is referred to as “2nd LINE.” The rightmost dot array is referred to as “12th LINE.” Reference numerals 1 to 9 indicate the positions of respective nozzles provided on the recording head 1.
FIG. 3B illustrates the recording timing reference signal (conveyance reference signal) 202 to be supplied to the head driving control unit 10A and the recording timing of the recording head 1. More specifically, FIG. 3B illustrates recording timing for dot arrays “7th LINE” and “8th LINE.” In FIG. 3B, an elapsed time Δt corresponds to a movement amount ΔX of the recording medium 2. According to the timing diagram of FIG. 3B, the recording head 1 operates to discharge ink from nozzles 2 to 8 for recording the dot array “7th LINE.”
For example, when the conveyance speed is set to 300 mm/s and a required recording resolution in the conveyance direction is 2400 dpi, the elapsed time Δt is 35.28 μS ((25.4 mm÷2400 dpi)÷300 mm/s). An actual recording apparatus includes a plurality of recording heads arrayed in the conveyance direction and is configured to adjust the drive timing of each recording head according to the distance between recording heads.
An example drive timing control operation is described below with reference to FIGS. 4A and 4B. FIG. 4A is a block diagram illustrating the timing generation unit 11A. FIG. 4B is a timing diagram of the timing generation unit 11A. The rotation pulse signal 201 has a conveyance resolution of 300 dpi, and the conveyance reference signal has a conveyance resolution of 1200 dpi.
In FIG. 4A, a rise detection unit 161 detects a rotation pulse signal 201 that changes from “0 (low level)” to “1 (high level)” and generates a rise detection signal 261. A counter unit 162 receives the rise detection signal 261 from the rise detection unit 161 and generates a count value 262 that represents the interval between two rise detection signals 261 successively input to the counter unit 162. A pulse width value holding unit 163 holds the count value 262 and generates a pulse width value 263. A conveyance reference signal pulse period calculation unit 164 calculates a value that corresponds to ¼ of the pulse width value 263. The conveyance reference signal pulse period calculation unit 164 generates a conveyance reference period value 264. A conveyance reference signal generation unit 165 generates a conveyance reference signal 202 based on the conveyance reference period value 264.
When the rotation pulse signal 201 changes from “0” to “1”, the rise detection signal 261, including a pulsative change, is generated from the rise detection unit 161. The counter unit 162 resets the count value 262 to 0 in response to the input pulse. The counter unit 162 starts a counting operation according to a clock signal that is, for example, a system clock of the system. When the rotation pulse signal 201 changes from “0” to “1” (when the rise detection signal 261 causes a pulsative change), the pulse width value holding unit 163 stores the count value 262 generated from the counter unit 162.
The count value 262 is, for example, the number of clocks used in the counter unit 162. While the counter unit 162 stores the count value 262 into the pulse width value holding unit 163, the counter unit 162 resets the count value to 0 and restarts counting for the next pulse. The conveyance reference signal pulse period calculation unit 164 generates the conveyance reference period value 264, which corresponds to ¼ of the pulse width value 263 (the value stored in the conveyance reference signal pulse period calculation unit 164).
The conveyance reference signal generation unit 165 receives the conveyance reference period value 264. The conveyance reference signal generation unit 165 and the counter unit 162 use the same clock signal. The conveyance reference signal generation unit 165 generates the conveyance reference signal 202 (a pulse signal) in response to each input of the conveyance reference period value 264. By repeating the above-described processing, the timing generation unit 11A generates the conveyance reference signal 202, having a conveyance resolution of 1200 dpi, based on the rotation pulse signal 201, having a conveyance resolution of 300 dpi. As described above, the recording apparatus can perform image formation processing based on the rotation pulse signal 201 obtained from the rotary encoder 8.
However, if the conveyance belt 3 causes a change in traveling speed due to eccentricity of the conveyance roller 4, the above-described control based on a rotary encoder is subjected to adverse effects. Similarly, if the conveyance belt 3 has irregularity in thickness, the conveyance belt 3 causes a change in traveling speed and, therefore, the above-described control is subjected to adverse effects.
FIG. 7 is a cross-sectional view of a conveyance roller 4 having a center 401 at a position offset from the center 400 of a rotational shaft. In FIG. 7, “r” represents the radius of the conveyance roller 4 and Δr represents an amount of eccentricity (distance between the center 401 of the conveyance roller 4 and the center 400 of the rotational shaft).
For example, the conveyance roller 4 has a radius “r” of 8 mm and the amount of eccentricity Δr equal to 5 μm. The conveyance roller 4 rotates at a constant speed, which is equal to 100 mm/S at the outer peripheral edge thereof. In this case, the radius of rotation of the conveyance roller 4 increases and decreases by an amount of the eccentricity Δr. The actual speed of the conveyance roller 4 at the outer peripheral edge thereof possibly varies in the range of 100±0.0625 mm/s.
FIG. 8A illustrates a dot position deviating from a target position when the recording apparatus illustrated in FIG. 2 performs image formation under the above-described conditions. In FIG. 8A, the abscissa axis indicates the position in the conveyance direction and the ordinate axis indicates the amount of positional deviation in a recording operation of dots on a recording medium at the interval of 486.8 μm. As understood from FIG. 8A, the dot position periodically deviates at the interval of approximately 50.3 mm. The interval of approximately 50.3 mm is equivalent to a circumferential length L (=2*π*r=2*8 mm*π=50.27 mm) of the conveyance roller 4 having a radius “r” of 8 mm.
As described above, among variations occurring in a transmission path from the conveyance roller shaft 6 to the recording medium 2, manufacturing errors (eccentricity, roundness, etc.) of the conveyance roller 4 have large effects (deterioration) on an image, which cannot be managed by the recording apparatus illustrated in FIG. 2. FIG. 8B illustrates an example change in speed of the recording medium 2 measured when the conveyance roller 4 having an outer peripheral length of 50.8 mm rotates at a constant angular speed of approximately 125 rpm to continuously convey the recording medium 2.
FIG. 8B illustrates a variation rate of the recording medium speed V relative to an ideal value (=50.8×125/60≈105.8 mm/sec). If the recording medium 2 is conveyed at an ideal speed, the speed change rate is equal to 0[%].
However, as understood from FIG. 8B, a large variation in the speed occurs at the period of 0.48 sec. An ideal relationship between the speed V of the recording medium 2 and the outer circumferential length L of the conveyance roller 4 is defined by L/V=50.8/105.8=0.48. The period of 0.48 sec corresponds to one complete rotation of the conveyance roller 4. It is understood that the speed variation at the period of 0.48 sec occurs due to eccentricity of the conveyance roller 4. The speed change of the recording medium 2 illustrated in FIG. 8B includes not only the above-described variation occurring at the period of 0.48 sec but also other components, such as speed changes caused by slippage between the recording medium 2 and the conveyance roller 4 or caused by other factors not related to the eccentricity of the conveyance roller 4.
In this manner, even when the conveyance roller 4 rotates at a constant angular speed, the recording medium 2 may not travel at the same conveyance speed. More specifically, the relationship between an output of the rotary encoder 8 that detects an angular displacement of the conveyance roller 4 and a conveyance distance of the recording medium 2 is not constant and is variable.
Accordingly, if an output pulse of the rotary encoder provided on the conveyance roller 4 is used to control the conveyance amount of the recording medium 2, the conveyance amount of the recording medium 2 includes an error component. A similar problem occurs when the conveyance motor 7 is a pulse motor that drives the conveyance roller 4, because the conveyance amount of the recording medium 2 is determined according to the number of pulses input to the pulse motor.
A similar problem occurs in a recording apparatus configured to convey a recording medium with rollers (e.g., serial type recording apparatus). The serial type recording apparatus alternately performs scanning/recording of a recording head and conveyance of a recording medium. If any error occurs in the conveyance amount of a recording medium during a recording operation, the positional deviation between a dot recorded before a conveyance operation and a dot recorded after the conveyance operation becomes large. If the conveyance amount is large, a recorded image includes a white streak. If the conveyance amount is small, a recorded image includes a black streak. In any case, the quality of a recorded image deteriorates.
A second method is described below. The second method uses a transfer belt on which a scale is provided to detect a movement of the transfer belt. As discussed in Japanese Patent Application Laid-Open No. 2004-287337, to eliminate irregularity in conveyance speed of a transfer belt, a recording apparatus can measure the speed of the transfer belt with an optical sensor that can detect (or read) a scale provided on the transfer belt. The recording apparatus can control a driving motor based on a measured speed so that the belt can travel at an optimum speed.
However, if the conveyance belt is unclean, the second method cannot accurately read the moving speed of the conveyance belt. For example, a conveyance belt in an inkjet recording apparatus tends to be unclean due to small ink droplets and paper dust. Similarly, a conveyance belt in an electrophotographic recording apparatus tends to be unclean due to residual toner and paper dust.
Next, a third method is described below. The third method uses a laser Doppler sensor (laser Doppler speed sensor). FIG. 5 illustrates a recording apparatus including a laser Doppler sensor 9 that can detect a movement amount of a conveyance belt.
The recording apparatus illustrated in FIG. 5 is different from the recording apparatus illustrated in FIG. 2 in that the rotary encoder 8 is replaced with the laser Doppler sensor 9.
FIG. 6 illustrates an example laser Doppler sensor 0300. A laser light source 0301 emits a laser beam LA. A beam splitter 0302 divides the emitted laser beam LA into two beams. One laser beam L1 passes through the beam splitter 0302 and reaches a measurement target 0310. The other laser beam L2 reflects from the surface of the beam splitter 0302 and, after having one more reflection from the surface of a reflection mirror 0303, reaches the measurement target 0310. Two laser beams L1 and L2, as illustrated in FIG. 6, are symmetrically incident on the measurement target 0310 at the same angle θ with respect to a perpendicular of the measurement target 0310.
The above-described laser beams L1 and L2 reach the measurement target 0310 moving at a speed V and become scattering light LB. The scattering light LB passes through an optical system (such as a collective lens 0304) and reaches a light-receiving sensor 0305. The light-receiving sensor 0305 detects the above-described scattering light LB and performs photoelectric conversion. An amplifier 0306 receives a converted electric signal from the light-receiving sensor 0305. The amplifier 0306 amplifies the amplitude of the input electric signal and outputs an amplified signal to a band-pass filter 0307. The band-pass filter 0307 generates a Doppler signal D (analog signal) based on heterodyne detection. A signal processing circuit 0308 receives the Doppler signal D from the band-pass filter 0307.
The signal processing circuit 0308 converts the above-described Doppler signal D having a frequency fD into a pulse signal having the same frequency fD. The signal processing circuit 0308 outputs the converted pulse signal as a speed signal.
The pulse signal has a period T that indicates a time required for the measurement target 0310 to travel a constant distance L. As discussed in Japanese Patent Application Laid-Open No. 2004-088416, the laser Doppler sensor 0300 can accurately detect a moving target without using any scale and can immediately generate a pulse signal representing the moving speed of the target to be measured.
Although the laser Doppler speed sensor 9 can directly measure the speed of a conveyance belt, the laser Doppler speed sensor 9 has the following problems. The laser Doppler speed sensor 9 does not operate stably because the Doppler signal suddenly attenuates sometimes and, therefore, the laser Doppler speed sensor 9 cannot accurately measure the speed of a conveyance belt in such durations.
FIG. 9A illustrates an example pulse waveform generated by the laser Doppler speed sensor 9. FIG. 9B illustrates plots indicating rise/fall edges of the pulse waveform illustrated in FIG. 9A. The Doppler signal attenuates in the duration from 200 μsec to 800 μsec, which is referred to as “dropout”, during which the laser Doppler speed sensor 9 cannot accurately detect the moving speed of a conveyance belt.